In electrostatographic applications such as xerography, a charge retentive surface is electrostatically charged, and exposed to a light pattern of an original image to be reproduced to selectively discharge the surface in accordance therewith. The remaining pattern of charged and discharged areas on that surface form an electrostatic charge pattern (an electrostatic latent image) conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder referred to as "toner". Toner is held on the image areas by the electrostatic charge on the surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate (e.g., paper), and the image affixed thereto to form a permanent record of the image to be reproduced. The process is well known, and is useful for light lens copying from an original, and printing applications from electronically generated or stored originals. The process has analogs in other electrostatographic applications such as, for example, ionographic applications, where charge is deposited on a charge retentive surface in accordance with an image stored in an electronic form.
It is common practice in electrophotography and its analogs to use wire corona generating devices to provide electrostatic fields driving various machine operations. Thus, corona devices are used to deposit charge on the charge retentive surface prior to exposure to light, to implement toner transfer from the charge retentive surface to the substrate, to neutralize charge on the substrate for removal from the charge retentive surface, and to clean the charge retentive surface after toner has been transferred to the substrate. These corona devices normally incorporate at least one fine wire coronode held at a high voltage to generate ions or charging current to charge a surface closely adjacent to the device to a uniform voltage potential, and may contain screens and other auxiliary coronodes to regulate the charging current or control the uniformity of charge deposited. The devices may be driven with positive or negative D.C. potentials.
Dicorotrons are A.C. driven corona devices incorporating a dielectric coating over the active coronode structure, to provide an arrangement which has the characteristics of an array of capacitors coupled to the air between the voltage source and the charge retentive surface, blocking D.C. current flow from the coronode to the charge retentive surface. The charging current to the charge retentive surface at any point on the coronode surface is limited by the maximum displacement current that can be delivered by the dielectric, which provides an essentially self regulating device. Applying a large A.C. potential to the coronode creates a gaseous plasma at the coronode that is maintained by the displacement current at any point. If the plasma current exceeds the displacement current at any point along the coronode, such as in the case of a non-uniformity caused by dust or debris on the coronode, the plasma potential drops and the plasma current is quenched at that point. If the plasma current is too low, the plasma potential rises and the plasma current is forced to increase. As a result of this action, the overall discharge from the coronode to the surface tends to be uniform since each point on the coronode surface delivers the same net charge per unit area to the plasma as every other point during each voltage reversal. The current extracted from the plasma to charge the charge retentive surface is therefore uniform and the device tends to be stable because of its self regulating behavior. By contrast, D.C. driven bare wire devices such as corotrons have a tendency to arc at non-uniformities along the coronode which, in effect, causes each point along the coronode to compete for current at the expense of adjacent areas. Thus, non-uniformity has an effect of reducing the available corona current along the entire coronode. This effect has a tendency to be more pronounced in negative charging devices.
Because of the problems, inherent in the use of suspended coronode charging devices, a significant amount of work had been done to provide charging devices without such a requirement. Among these are U.S. Pat. No. 4,057,723 to Sarid et al., which shows a dicorotron having a dielectric coated coronode uniformly supported along its length on the shield or an insulating substrate. U.S. Pat. No. 4,409,604 to Fotland teaches support for a cylindrical, dielectric coated coronode in an insulating support. U.S. Pat. No. 4,155,093 to Fotland et al. shows the use of two electrodes separated by a dielectric, in combination with a conductive support with a dielectric coating to receive extracted ions. Published Japanese Patent Application No. 58-48073 to Momotake, teaches a charging device for selectively charging portions of the image area of a photoreceptor, having rectangular discharging electrodes supported on a glass insulator. U.S. Pat. No. 4,430,661 to Tarumi et al teaches an ion modulation arrangement which modulates the direction of ion flow directed therethrough from a wire corotron device.